1. Field
Exemplary embodiments of the present invention relate to a memory device.
2. Description of the Related Art
As the integration degree of memories increases, the gap between word lines that are in the memories decreases. The narrowing of the gap between word lines increases coupling effects between neighboring word lines.
Whenever data is inputted/outputted to/from a memory cell a word line toggles between an active state and an inactive state. However, as the coupling effect between the neighboring word lines increases, as mentioned above, the data of memory cells coupled with the word lines disposed adjacent to frequently activated word lines may be damaged or lost. This phenomenon is referred to as word line disturbance. Word line disturbance may damage data of memory cells before the memory cells are refreshed.
FIG. 1 illustrates a portion of a cell array included in a memory and describes a word line disturbance effect. The drawing describes the word line disturbance phenomenon.
In FIG. 1, a reference symbol ‘WLK’ represents a word line that is frequently activated, and reference symbols ‘WLK−1’ and ‘WLK+1’ represent word lines disposed adjacent to the frequently activated word line WLK. Also, reference symbol ‘CELL_K+1’ represents a memory cell coupled with the frequently activated word line WLK, and reference symbol ‘CELL_K−1’ represents a memory cell coupled with the word line WLK−1. Reference symbol ‘CELL_K+1’ represents a memory cell coupled with the word line WLK+1. The memory cells CELL_K, CELL_K−1 and CELL_K+1 include cell transistors TR— K, TR_K−1 and TR_K+1 and cell capacitors CAP_K, CAP_K−1 and CAP_K+1 respectively.
In FIG. 1, when the frequently activated word line WLK is activated or deactivated, the voltages of the word line WLK−1 and the word line WLK+1 rise or drop due to the coupling effect occurring between the word line WLK and the word lines WLK−1 and WLK+1, affecting the amount of charge stored in the cell capacitors CAP_K−1 and CAP_K+1. Therefore, when the word line WLK is frequently activated and thus the word line WLK toggles between the active state and the inactive state, the change in the amount of charge stored in the cell capacitors CAP_K−1 and CAP_K+1 included in the memory cells CELL_K−1 and CELL_K+1 is increased, thus deteriorating the data of the memory cells CELL_K−1 and CELL_K+1.
Also, electromagnetic waves generated while the word line WLK toggles between the active state and the inactive state induces the movement of electrons in or out of the cell capacitors of the memory cells coupled with the neighboring word lines WLK−1 and WLK+1, thus damaging the data of the memory cells.
The phenomenon that a particular word line, e.g., the word line WLK, is repeatedly activated more than a predetermined number of times and thus the data of the memory cells coupled with the neighboring word lines, e.g., word lines WLK−1 and WLK+1, are affected by the frequently activated word line WLK is called ‘a row hammer effect’. Among the methods used to address the row hammer effect is a method of performing an active operation on the neighboring word lines WLK−1 and WLK+1 disposed adjacent to the frequently activated word line WLK. Through the active operation performed on the neighboring word lines WLK−1 and WLK+1, the data of the memory cells coupled with the neighboring word lines WLK−1 and WLK+1 are programmed again, preventing the data from being damaged. However, such a method used to address the row hammer effect may not be applied to redundancy word lines, and this is described below with reference to FIG. 2.
FIG. 2 illustrates word lines WL0 to WL511 and redundancy word lines RWL0 to RWL7. FIG. 2 shows a case where a word line WL3 is replaced with a redundancy word line RWL2.
When a word line WL3 is replaced with a redundancy word line RWL2, as shown in FIG. 2, and the word line WL3 is repeatedly accessed more than a predetermined number of times, the redundancy word line RWL2 is actually activated more than the predetermined number of times. In this case, activating the neighboring word lines WL2 and WL4, which are disposed adjacent to the word line WL3, does nothing for the prevention of the row hammer effect. This is because the memory cells that are likely to lose their data due to the frequent access to the word line WL3 are not the memory cells coupled with the word lines WL2 and WL4, but the memory cells coupled with redundancy word lines RWL1 and RWL3.